This invention relates to a digital signal process apparatus for use in a video tape recorder (VTR), and more particularly to a signal process apparatus for converting a digital signal into a scrambled recording signal format.
In recording a digital video signal, it is well known that NRZ (Non Return to Zero) modulation is effective since its frequency bandwidth is relatively narrow and it is therefore suitable for high density recording. However, since its maximum transition is infinite, a long period of continuous "0"s or "1"s may bring about a lack of reproduced pulses, thus involving difficulty in clock recovery. Accordingly, signal randomization (scrambling) and block-coding are advantageously employed as an available method for limiting the continuous repetition of "0"s or "1"s. However, when a comparatively correlative signal such as a video signal is recorded, specific code patterns are frequently produced which causes a word error, thus deteriorating into a whole error rate. This is well known as a pattern effect. To reduce the pattern effect, the digital video signal is randomized by scrambling, even in cases where block-coding is used. Further, in order to restore an original video signal from a reproduced signal which contains errors arising from so-called drop-out or jitter, a synchronizing code and a check code are added to the digital video signal. In this case, the digital video signal is divided into units each having 200 to 1000 bits, and the synchronizing code and the check code are added to each unit. One unit having the synchronizing code and the check code is treated as one sub-block, and an error correction process is carried for each sub-block.
A jitter of time base is included in a VTR-reproduced signal and corrected by a time base corrector (TBC). In the case of normal speed reproduction, it is enough to have a time base correction range of 2 to 3 sub-blocks for cancelling the jitter in the TBC. However, a time base correction range of several to ten sub-blocks is necessary for shuttle reproduction and slow motion reproduction. Accordingly, two kinds of synchronizing code inserted in the video signal are needed, i.e. one is added for each sub-block and the other is added for a long period which corresponds to the time base correction range of several to ten sub-blocks.
In a prior art system, a random signal process is performed by adding a pseudo-random signal to an input digital video signal in a manner of MOD 2, while the cycle period of the pseudo-random signal corresponds to a synchronous period, i.e., the period of the sub-block or the period of several to ten sub-blocks. However, when the video signal is randomized by using the cycle period of the sub-block, a satisfactory effect is not ensured with regard to the pattern effect, and when the video signal is randomized by the cycle period of the several to ten sub-blocks, the failure to detect the synchronizing code may lead all data into an error condition until the next synchronizing code is correctly detected. Accordingly, there is no suitable scrambling method, which is free from both the pattern effect and the error spread.